560 VITAL ACTIONS OF THE LOWER FUNGI. 



series, such as tyrosine, phenol, cresol ; reduction pro- 

 ducts, such as indol, hydro-paracumaric acid ; complex 

 molecules, such as carbo-hydrates, peptone, hydrolytic 

 ferments ; finally, colouring materials and alkaloid-like 

 poisonous substances. According to the species of the 

 chief fungus present, and according to the external con- 

 ditions of the nutrient medium, sometimes the one, 

 sometimes the other of these products appear, and a 

 particularly large number of them is observed when we 

 have to do with fermenting or putrefying substrata. 



Among these products of tissue change we can sepa- 

 rate a group which generally occurs in the vital processes 

 of all, or at any rate of numerous fungi ; and we can place 

 in opposition to this the group of the rarer and more 

 specific products, the occurrence of which is limited to 

 one or a few species. It is also of special interest to 

 ascertain in how far the number and the quality of the 

 products of each individual species of fungus undergo 

 variations according to differences in the composition of 

 the nutrient substratum, and whether, on the other 

 hand, when the nutrient substratum is the same, the 

 same products are always furnished by the same species 

 of bacteria ; in other words, whether the power of each 

 species of breaking up a given nutrient substratum in a 

 particular manner is constant and special. It is clear 

 that the answer to these questions is of extreme 

 significance for the diagnostic value of the products of 

 the tissue change. 



Carbonic acid. Carbonic acid alone is of constant occurrence ; water 

 and a nitrogenous body also undoubtedly always appear 

 as products of decomposition, but they do not, like the 

 carbonic acid, come under observation as excreta. The 

 life of the lower fungi seems never to go on without 

 separation of carbonic acid ; hence it is always present, 

 although in very varying quantities, whether it arises 

 only as the result of intra-molecular respiration, or 

 whether oxygen has been present, or fermentation has 

 occurred. 



Fatty acids A production of fatty acids and oxy- acids (acetic acid, 

 propionic acid, butyric acid, lactic acid, succinic acid, 



